1. Field of the Invention
This invention relates to electronic circuits for braking alternating current induction motors and more particularly, to such circuits for braking motors for driving power tools.
2. Description of the Prior Art
Many types of machinery including power tools are driven by electrical motors, which are energized for performing a machining operation and thereafter, de-energized so that a further machining step may be performed. In such machines, it is usually essential that the prime mover illustratively in the form of a alternating current induction motor be stopped quickly and precisely, so that there will be no significant over-travel. For example, alternating current motors are used to drive radial arm saws; after the motor is deenergized, it may require two or three minutes for the motor driven saw blade to "coast" before the saw blade is brought to a stop. During this coast down period, the operator must avoid inadvertent contact with the rotating blade for obvious safety reasons. The operator also is restricted from re-adjusting the saw blade until it comes to a complete stop. As a result of the coasting or over-travel, productivity is reduced.
Therefore, it is desirable to employ a braking mechanism so that machinery will stop almost immediately when desired, and over-travel will be eliminated or at least reduced to non-significant intervals. Mechanical braking mechanisms are known in the art and rely primarily upon the engagement of two frictional members, such as a brake drum and brake band, or a clutch disk and clutch plate. This type of braking device, being essentially mechanical, requires a time interval to bring the machinery to a stop. More over, mechanical braking devices wear rather rapidly and, therefore, require frequent adjustment, maintenance and repair.
Due to these deficiencies of mechanical brakes, electrical or electronic circuitry has been employed for braking alternating current induction motors. A summary of various forms of electrical brakes is set forth in International Rectifier News for February-March 1957 in an article entitled "Direct Current Braking for AC Induction Motors." Known methods include applying electrical power in reverse phase rotation to develop a reverse torque, dynamic braking in which a resistive load is shunted across the motor terminals, capacitor braking in which capacitors are connected across two or three phases of an induction motor; re-generative braking applied to a motor driven above its synchronous speed, and direct current braking effected by applying a direct current to the field winding of an alternating current motor. This invention relates to a new and improved form of direct current braking of an alternating current induction motor.
As well recognized in the prior art, direct current braking employs the principle of removing an alternating current from rotatively energizing the motor and, in order to effect braking, applies an amplitude and time controlled DC voltage across the winding resulting in a static field across the stator which generates in the rotor of the motor a counterforce which cooperates with the static field to decelerate the rotor to a point of zero differential relative velocity with the static field. Such direct current or dynamic braking is achieved solely by electrical means and requires no mechanical modification to existing motors or equipment.
U.S. Pat. No. 4,243,919 of Brown is an example of braking an alternating current induction motor by applying DC pulses via a silicon controlled rectifier (SCR) through the field winding of the alternating current induction motor. In the Brown patent, a single SCR is turned on for an interval controlled by a simple transistor charged RC timing circuit. In particular, the SCR is rendered conductive during every positive half-cycle of the AC voltage to apply a half-wave rectified DC voltage to the motor field winding. A switch disclosed as being coupled to a foot controller of the motor is associated with a circuit for sensing the opening of the switch to initiate timing of the RC timing circuit, the timed interval being set to permit the motor to be completely braked. The output of the RC timing circuit is coupled to a gate of the SCR to initiate its firing.
U.S. Pat. No. 4,195,255 of Guttmann also discloses an SCR braking system for alternating current induction motors, wherein an SCR is turned on for a period of time to apply during that interval DC pulses in a manner to brake the motor. In particular, the SCR conduction time is made adjustable by means of a potentiometer.
Further, the prior art teaches the control of the braking force applied to alternating current induction motors by phase controlling a switching device such as an SCR or thyristor that is coupled to the motor's armature. U.S. Pat. No. 3,897,595 of Fearno discloses not only the adjustment of the braking time, but also the adjustment of the braking current as applied by a switching device in the form of an SCR to the motor. In particular, Fearno discloses a braking contactor having braking contacts for connecting a rectifier means to the AC source of power, the rectifying means taking the form of an SCR for varying the amount of current conducted or rectified through the stator field winding of the motor. A free wheeling diode is also connected by the closed braking contactor across the stator field winding in an oppositely polled manner from the SCR to absorb counter EMF generated in the field winding as the static field is applied to the rotor and a current is generated in the shorted conductors therein. In particular, a force or braking current circuit is connected to the gate of the SCR, whereby the firing of the SCR may be selectively controlled for a portion of each half-cycle of the AC voltage to vary the amount of current, i.e. the width of each DC pulse, applied to the stator field winding. A light source is coupled to the AC voltage applied to the stator field winding and serves to initiate the braking action when extinguished. When AC voltage is removed from the stator field winding, the light source darkens whereby the resistance of an optically coupled diode is increased to initiate the timing of an interval during which the DC current pulses are applied via the SCR to the stator field winding. The optically coupled diode also serves to energize the braking contactor to close the braking contact thus completing the circuit between the stator field winding and the SCR. The time duration during which braking current is applied by the SCR to the stator field winding and controlled by a further potentiometer.
Though the Fearno patent discloses that the SCR is fired only after his contactors are closed, it is apparent that a load is placed upon these contactors drawing current therethrough thus requiring relatively heavy duty motor contactors, which components are relatively expensive. Further, though an optical switch is provided by the use of the light source for detecting the energization and de-energization of the stator field winding, it is apparent that other circuitry is directly connected to the SCR as well as to the energizing or actuating coil of the braking contacts. Thus, it is possible for transients as would be applied by the three phase energizing lines to be imposed upon the Fearno's motor brake circuit.
In addition, the Fearno patent discloses the use of an RC circuit to control the conduction interval or angle that its SCR is turned on. In particular, the RC circuit comprises a potentiometer which is set to determine the conduction angle. However, the Fearno RC circuit limits the braking capabilities in that it is only able to fire its SCR for maximum angles of 90.degree., thereby limiting the conductive angle and thus the current applied to the winding of the motor to be braked.
Further, the Fearno patent discloses the firing of the SCR dependent upon the setting of a potentiometer, whereby once this potentiometer is set, the amount of current conducted or rectified through the stator field winding is set. By contrast, this invention contemplates the tailoring of the current pulses applied to the stator field winding in a manner that the braking torque is set initially high followed by a period in which the braking torque is reduced. Thus, it is possible to initially reduce the speed of the motor rapidly by applying high current (large conduction angle) DC pulses via an SCR to a motor's winding, followed by a period of reduced braking torque wherein the conduction angle of firing the SCR is reduced to thereby reduce the current applied to the stator field winding. In this manner, a maximum braking torque may be applied to a particular motor dependent upon the mechanical and electrical characteristics of the motor for a first brief period. However, if the maximum braking torque were applied over a prolonged period, the inertial stresses placed upon the motor as well as any mechanism or tool coupled thereto could be excessive. For example, if a radial saw were coupled to a motor, it is contemplated that the rotational forces applied to the saw blade may cause the saw blade to unloosen its retaining bolt, whereby the saw blade may be thrown from the motor with possible damage to the operator. Thus, it is desired, in accordance with the teachings of this invention, to reduce the braking torque in order to prevent damage to the motor or associated mechanism, the subsequent braking torque and interval thereof being dependent upon the nature of the mechanism or tool coupled to the motor.
Further, it is contemplated that the electronic motor brake of this invention is adaptable for the control of a wide range of motor sizes and voltage ratings including both single and three phase AC voltages. In this regard, it is necessary to set the maximum braking current dependent upon the particular type of motor and its electrical characteristics. Further it would be necessary to accommodate the particular energizing voltage, which may illustratively assume a value of 120, 220 or 440 volts or be single or plural phase.